Phage Wars 2

Imagine, for a moment, that you - yes, you - are a biologist. You're not just any biologist, either: You're engineering a virus - the nastiest virus imaginable. Why? Who knows! Maybe it's revenge, maybe you're just curious, maybe you want to take over the world - fill in the blanks.

In order to design this ubervirus, you need to ... destroy all the other viruses in the lab. Infect healthy cells which in turn generate more viruses, attack the enemy virus while protecting your own cells - once you've spread through the entire map and have utterly obliterated your rival, victory is yours! After every victory you discover a new genome sequence, allowing you to boost your virus' rate of movement, its reproductive speed, its defense and its attack, making you more formidable in battle.

Ok, I'm actually reasonably sure that this isn't how viruses actually work, but Armor Games' Phage Wars 2 is a damn fun strategy-action hybrid. In later levels, when you're dealing with multiple enemy viruses and other factors like cancerous cells (reproduces viruses at a much higher rate than normal) and bacteria (... not sure what they do, exactly), it's very hectic and hard to keep track of. In a fun way, though.